Field effect transistors are well known in the art, including various gating techniques. An enhancement mode FET conducts current between its drain and source in response to gate voltage. Because of the capacitance that exists between the gate, source and drain of the FET, any change in the gate voltage is achieved only through an attendant movement of charge to and from the FET gate region. The speed with which a FET can be turned ON and OFF is dependent upon the speed with which the charge can be stored in and removed from the gate capacitance. Some gating circuits are able to supply sufficient to charge the gate rapidly to attain fast turn-on, but must rely on a resistance connected between the gate and source of the FET to remove the gate charge for turn-off. In order to achieve fast turn-off a low value resistance must be used requiring that a high current be maintained through the gate resistance while the FET is ON, all as is known.
The present invention provides fast turn-off but without a high current required from the gate driving source.
The FET turn-off circuitry of the present invention is self-powered in that the gate to source capacitance of the FET is used to drive a nonregenerative switch into conduction to enable faster discharge therethrough of the FET gate, whereby to facilitate faster FET turn-off.